The conventional roller-type liquid applicator has a roller, which is dipped in a liquid such that the liquid is carried on the surface of the roller so as to facilitate the applying of the liquid on a workpiece. The conventional roller-type liquid applicator is in fact rather primitive at best in terms of the way it works. It is conceivable that the liquid carried on the surface of the roller may drip in drops on a surface other than the surface of the workpiece, and that the liquid can not be evenly applied on the surface of the workpiece. The most serious drawback of the conventional roller-type liquid applicator is that it is inefficient, especially at the time when it is used to apply the liquid on the surface of a large area.
The U.S. Pat. No. 4,576,553 discloses a technique by which the liquid is pumped into the roller of a roller-type liquid applicator. However, the internal structure of the roller is rather complicated. In addition, the pump that is used to pump the liquid into the roller must be specially designed. Moreover, the applicator must be used in conjunction with a throttle control switch so as to regulate the flow at which the roller is provided with the liquid. The throttle control device may be replaced by an expensive pump designed to feed the liquid at a constant flow rate. It is therefore readily apparent that such a prior art roller-type liquid applicator as described above is not cost-effective at best.